I just finished building (about 3 months ago) an Ar15 upper in 300-221 fireball aka 300 Whisper.
You should look into other sources for your barrel. You can get a very high quality AR barrel from Randall at AR15 Barrles.com.
http://www.ar15barrels.com/
You can also get a barrel form Tony at Del-Ton. He bulit a very nice 16 in. upper for me. The barrel is an Olympic Arm barrel built for them by wilson barrels. I am having Randall at AR15 Barrles thread my 16 in barrel for me.
www.del-ton.com
Another source is
www.model1sales.com. But there was some scuttle on the web that their 1 in 8 barrels were not really 1 in 8.
Here are my suggestions:
-16 in barrel
-1 in 8 twist
-gas port in pistol position
-adjustable gas block... I got mine from JP Enterpises:
www.jprifles.com
-you will have to learn to load your own ammo
-a good source of brass:
keith__davis@yahoo.com
-The 300 will be very accurate out to and beyond 200-300 yds
-Do not limit yourself to only subsonic loads
-The case capacity of the 300-221 fireball is similar to the 357 mag. but it can run a 52000 psi
-Hodgdon H110 is a good powder for super sonic loads. Hodgdon strongly recomends that you do not use it in for subsonic/reduced loads. H110 can be temp sensitive and pressures can vary. Here is a URL to a persons site that documents blowing up a barrels usnig light loads of H110:
http://feistyrooster.com/300x221/300x221index.html
-Hodgdon Lil'Gun is a great power for super sonic loads.
-Accurate Arms has several subsonic loads listed. I have shot AA#9 for my subsonic loads and it was very consistent. Check my posts... I have listed the Accurate Arms loads on this site already. You can also go to the Accurate web site and get them there.
-Sierra Bullets has several loads listed. Contact them and they will send you pdf file with the loads... 110 gr to 240 gr bullets.... I have tried 125, 130, 150, 165, 175, 180, and 200 gr Sierra bullets and Nosler balistic tips and Speer 165. All run well. The Noslers tended to give higher pressures with lower powder charges than the Speer and Sierra bullets.
I do not have much experience with cans... can't help you with that
If you have any questions about your project you will find a lot of people will to share their experiences here.
Have fun building your porject